In some applications, such as aircraft applications, batteries are used as a power source for engine starting or as an emergency power source for a DC bus. Obviously, it is important to maintain the battery in a fully charged condition and to charge it after use. In known systems, a DC bus to which the battery is connected may itself be powered by a DC source, such as an engine-driven AC generator and a transformer rectifier unit (TRU) to convert AC to DC. During normal operation, the DC power source provides power for electrical loads of the system, in addition to providing power for charging the battery. If the DC power is interrupted, the battery immediately provides power to the DC bus. In order to reduce the voltage variation on the DC bus, the battery nominal voltage is usually selected to be close to the DC bus voltage.
The battery system with these components is simple and low cost so it is widely used in aircraft, electrical vehicle, and telecommunications applications. However, in this configuration, the battery is maintained in a float mode where the battery is fully charged and is essentially being topped off continuously because the DC power feeding the bus almost always is available. Therefore, the battery may be overcharged for long periods of time, resulting in battery overheating and electrolyte loss. On the other hand, if the bus voltage is too low the battery could be undercharged, resulting in capacity fade. In both cases this results in reducing system reliability, as well as increasing battery system maintenance cost. In addition, it may be undesirable to keep a completely discharged battery floating on the bus because the battery may draw a large transient current during initial charging and overloading the DC source.
One approach to these problems is to regulate the DC bus voltage in a way that is better for charging the battery. For example, the DC bus voltage may be initially reduced following battery discharge to prevent large current in-rush transients. The voltage may then be raised above a normal charging level to increase the battery charging rate. Such systems provide better control over battery charging to prevent overheating of the battery, and allow the battery to be recharged more quickly. However, there is substantial increased cost and complexity in regulating the DC bus voltage, and other aircraft systems connected to the bus may be affected.
More often, the DC bus is powered with an unregulated source, such as a TRU. In these systems, attempts have been made to intermittently connect the battery to the bus, as described, for example, in U.S. Pat. No. 3,703,675, which uses a contactor to control the battery charge and a parallel diode to provide a discharge path, and U.S. Pat. No. 5,969,436, which uses a MOSFET in series with a diode in order to control battery discharging. Another approach has been to add a "boost" circuit in series with an intermittently charged battery, to provide a voltage higher than the DC bus voltage for battery charging. See, for example, U.S. Pat. No. 4,061,956. Also, U.S. Pat. No. 4,443,752 describes a similar technique using a MOSFET as a switch. In either of these systems, two series-connected power components are provided on the high current path between the DC bus and the battery, which may increase power dissipation during operation.